J. Agríe. Food Chem. 1990, 38,
18
18-24
Nitrogen to Protein Conversion Factor for Ten Cereals and Six
Legumes or Oilseeds. A Reappraisal of Its Definition and
Determination. Variation According to Species and to Seed Protein
Content
Jacques Mossé
Laboratoire d'Etude des Protéines, Département de Physiologie et Biochimie Végétales, INRA,
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78026 Versailles Cedex, France
The main two kinds of usual nitrogen to protein conversion factors {kA and kP) were investigated
from the amino acid composition of seed samples with widely varying N content for 10 cereal species
and for several legumes and oilseeds. The variations of these factors as a function of seed N content
were determined, and their values were compared with the few data from the literature. It was shown
that these two factors are the upper and lower limits, respectively, of the total seed N to true protein
conversion factor k, which is close to the average of kA and kP. This enabled determination of the
factor k with an improved reliability and also comparisons of k values either within a species for distant seed N contents or for a given N content between different species. The conversion factor k
varied from 5.1 for N-poor rice grains to 6.0 for N-rich foxtail millet grains. On an average per species, all other species ranged between those two extreme values. They are lower than generally acknowledged until
now.
value never valid in plant material, as it has been successively emphasized by Jones (1931), Heathcote (1950),
Tkachuk (1966b, 1969, 1977), Heidelbaugh et al. (1975),
Sosulski and Holt (1980), de Rham (1982), and Smith
(1987).
As early as 1896, for wheat grain, Teller (1932) selected
the factor 5.7, which is less inaccurate than that (5.83)
calculated by Jones (1926). Jones (1931) extended his
calculation to other species. He formulated clearly the
problem, so that the questionable values he suggested
remain still widespread today, in spite of various improvements successively made by Heathcote (1950), Kutscher
and Langnau (1965), Tkachuk (1966a,b, 1969,1977), Tkachuk and Irvine (1969), Ewart (1967), Holt and Sosulski
(1979), Sosulski and Holt (1980), and Morr (1981,1982).
Most of these authors determined the factors from more
valid AA data that became available during the 1960s.
The most accurate AA compositions ever published until
recent years are those of Tkachuk and Irvine (1969) who
determined AAs from five different hydrolysates per sample and those of Sosulski and Holt (1980). It is strange
that neither the FAO (1970) nor the WHO (1973), in their
data or recommendations, used the results of Tkachuk
(1969), which correspond to real progress. The same is
true for the AOAC (1980) who has been “seeking comments and supporting data on N:P conversion factors”.
However, Tkachuk and Irvine (1969) and Tkachuk
(1969) analyzed only one single (or two) composite sample^) per species, so that their results “could not be used
to indicate differences in AA composition as influenced
by variety or protein content”. This latter kind of research
has been undertaken during the last years in our laboratory for wheat, maize, pea, lupin, triticale, sorghum, rice,
and foxtail millet (Mossé et al., 1985; Baudet et al., 1986a;
Mossé et al., 1986, 1987a,b, 1988a-c, 1989), for rye (Baudet et al., 1987), for barley (Huet et al., 1988), and also
for oats, pearl millet, French bean, broad bean, soybean,
and sunflower seeds (unpublished results). For most of
these 16 species, the conditions of AA analysis were generally similar to those used by Tkachuk and Irvine (1969),
with six different hydrolysates per seed sample. More-
Today, cereal grains and legumes or oilseeds remain
by far the predominant source of protein used for human
food and for farm animal feed. It is thus important to
have an accurate knowledge of the protein concentration of grains. This becomes more and more indispensable in many areas: in human or animal nutrition, in marketing grain according to protein content, in the feed industry, in practical feeding of farm animals (mainly the
monogastric ones), and particularly in scientific experimental research requiring definite rations. Recent
advances in animal nutrition have revealed that excess,
as well as lack of, protein (or of a few particular essential amino acids (AAs) that change according to the animal species involved) can be detrimental. For instance,
not only is any excess of protein uneconomical and costly
but it is also harmful for environment due to the problem raised by feces elimination of monogastric animals.
The protein concentration of grain is obtained by multiplying its total nitrogen (N) content by a nitrogen to
protein (N:P) conversion factor calculated from the AA
composition of grain. In other words, the (true) N:P conversion factor k can be defined as the ratio of total protein content to total N content in grain (or in any other
biological product). On a theoretical basis this definition seems clear and simple. However, in fact, the analytical recovery of AA analyses cannot reach 100% and
N can be evaluated by different ways. Therefore, as discussed in the present paper, there are several ways to
assess the N:P conversion factor. Moreover, the values
used for this factor by many authors, including FAO (1970),
WHO (1973), and AOAC recommendations (Baker, 1979),
because
are partially conventional and frequently erroneous
they are based on inaccurate data and on forgotten assumptions. Otherwise, they are often mistaken for the inverse
of the N percentage of total proteins. Though this latter choice often coincides with the use of improved AA
of nondata, it remains erroneous, due to the occurrence
protein N (NPN). Moreover, the far most prevailing way
to express AA composition, even in nonnutritional research,
is as grams of AA per 16 g of total N: This indirectly
6.25, a
implies that the conversion factor is 100/16
=
0021-8561/90/1438-0018$02.50/0
©
1990 American Chemical Society
N-Protein Conversion Factor for Grains
J. Agríe. Food Chem., Vol. 38, No. 1, 1990
19
Table I. Description of Seed Analyzed and Conditions Utilized for Amino Acid Determination
% N in seed dm (N)
duration or purposes of hydrolyses
samples
cv.
samples/cv. (max)'
low
high
wheat
30
12
triticale
19
9
7
7
30
13
12
8
33
20
6
7
11
11
3
5
6
8
11
10
14
6
1.42
1.70
1.45
1.03
1.50
1.27
2.84
3.80
3
3
4.90
pearl millet
20
20
1
7
2
oats
13
56
21
17
24
24
1
7
7
5
10
1
1.10
1.82
1.02
3.73
2.61
3.39
1.29
3.27
3.06
4.01
2.95
2.97
2.07
5.15
7.75
6.94
3.34
3.65
3.80
6.04
5.39
4.89
4.37
species
barley
maize
sorghum
rice
pea
lupin
soybean
foxtail millet
broad bean
sunflower
French bean
rye
5
13
1
1
4
15 h
range
1.85 \
1.36 )
2.56
1.92 {
1.47 >
0.80 1
2.31 1
3.95 ]
48 h
18 h6
Trp
amide
ref
1
2
/
3
4
X
X
X
X
X
X
5
6
7
2.04/
2.24)
X
1.83 >
2.78 )
2.31
2.78
1.50
3.08
b
0
24 h
X
X
X
X
X
X
X
X
8
9
9
10
9
X
X
X
X
X
9
9
9
11
Highest number of samples per cultivar (cv.). After previous performic oxidation. Key: (1) Mossé et al. (1985), (2) Mossé et al.
(1988a), (3) Huet et al. (1988), (4) Baudet et al. (1986a), (5) Mossé et al. (1988b), (6) Mossé et al. (1988c), (7) Huet et al. (1987), (8) Mossé et
al. (1987b), (9) unpublished results, (10) Mossé et al. (1989), (11) Baudet et al. (1987).
c
over, on an average, about 20 samples per species were
selected from a number of varieties and for their wide
range of protein content.
The purpose of the present paper was to show that, in
the absence of perfectly accurate values of the conversion factor, it is still possible to accurately determine its
upper and lower limits, which are often close to each other;
to describe quantitatively the variations of these limits
as a function of grain N content; and within these limits, to assess improved values of this factor with a known
degree of uncertainty.
MATERIALS AND METHODS
Materials. The main characteristics of the materials
ana-
reported in Table I. They comprise seeds from 16
species: 10 cereals, 5 legume seeds (including soybean), and 1
oilseed. The number of samples analyzed varied from 5 for soybean to 33 for pea seeds. For several species about one-third
to half of the samples studied belonged to the same cultivar (or
variety, inbred line, or hybrid according to the species), in order
to investigate separately the influence of environment on AA
composition (i.e., phenotypic variation) while the other samples were selected from different cultivara in order to study the
possible influence of genome. For each species, the seed N percentage on a dry basis (N) spread over the widest possible range
(Table I).
Analytical Methods. Seed sampling, milling, meal subsampling, meal dry matter, and total N determinations and AA analyses were performed under conditions previously detailed elsewhere (Mossé et al., 1985). For seven of the species investigated, amide ammonium was not determined and for two of
them tryptophan was not analyzed, while the other AAs were
determined from two or three hydrolysates only. However, for
the nine other species AAs were determined from six idfferent
hydrolysates. In order to make allowance for losses resulting
either from partial degradation or from incomplete release, three
hydrolyses (15, 24, 48 h) were performed in boiling 6 M HC1 in
addition to an 18-h hydrolysis of a previously oxidized sample
for sulfur AAs. Amide NH3 from asparagine plus glutamine
was determined after a separate hydrolysis (3 h in 2 M HC1 at
115 °C) and tryptophan from an alkaline hydrolysate in
Ba(OH)2.
It must be emphasized that in addition to the utilization of
nonbiological AAs such as norleucine or a-amino-/3-guanidopropionic acid, which were added to the mixture of common AAs
(i.e., protein AAs) for calibration of the analyses, aliquots of a
long-term stored hydrolysate of a laboratory-defatted soybean
meal were used as standards for each ninhydrin preparation to
monitor the reproducibility. Moreover, commercial egg white
lysozyme (Merck) and purified human serum albumin (NBC)
lyzed
are
used as reference proteins to check the accuracy of AA
determinations. Thus, AA compositions used here probably represent the most complete analyses of the total proteins of cul-
were
tivated seeds.
Definition and Calculation of the Conversion Factor.
It has been previously emphasized (Mossé et al., 1985) that the
conversion factor can be defined in several ways. A first definition corresponds to the ratio
* = >··/ °<
(l)
of actual seed proteins (that is total AA residue weight) to total
N recovered from the 20 AAs (amide N of asparagine and glutamine being included). The calculation of kh necessitates the
use of two expressions of AA composition of seeds: (1) as grams
of residue (E¡) of the ith AA/100 g of seed dry matter (a residue is an anhydrous AA; i.e., the actual molecular fraction of
an AA bound and copolymerized in polypeptide chains); (2) as
grams of N (D¿) of the ¿th AA/100 g of seed dry matter. The
total weight of residues Y,Et includes the 20 common protein
AAs. Although data are always restricted, at the best, to 18
figures plus the amide NH3, the total of glutamic acid plus glutamine Glx = Glu + Gin is always determined because glutamine gives glutamic acid during acid hydrolysis by substitution of a carboxyl group (COOH) to its amide group (CONH2).
By chance, the MW values of Glu (147.13) and Gin (146.15) are
close enough to describe their total Glx in terms of Glu. The
same is true for the total of aspartic acid and asparagine (MW
133.11 and 132.12, respectively): Asx = Asp + Asn. It is obvious that neither the amide NH3 nor the analytical NH3, which
can be determined in seed meal hydrolysates, has to be included
in the total ££,· although some authors do so erroneously. Howof AA N includes the 19th figure:
ever, the total weight
that is, the amide N released as NH3 by asparagine and glutamine when they are converted by acid hydrolysis into glutamic and aspartic acids. For lack of specific amide N determination, the total NH3 determined from 6 M HC1 hydrolysates can be used (Heathcote, 1950; Tkachuk, 1969) by keeping
in mind that this total NH3 amount is generally equal to, or
sometimes slightly higher than, amide NH3. In order to be as
rigorous as possible, it is preferable to distinguish a factor kx'
(close to kA) when amide NH3 is replaced by hydrolysis NH3
in the total Y.D¡.
Another useful factor corresponds to the ratio
kr
to
total
proteins
=
</
(2)
of actual seed
seed N content in 100 g of seed
dry matter (N being determined by the micro-Kjeldahl method).
Another factor, kN, giving approximately the weight of the
main N compounds (proteins plus nucleic acids) from N has
J. Agrie. Food Chem., Vol. 38, No. 1, 1990
20
Mossé
Table II. Slopes (u or p, ±SD), Intercepts (vor q, ±SD), and Correlation Coefficients r of Regression Lines Representing
=
Total Amino Acid Content as a Function of Nitrogen Content (N), as Grams of Amino Acid Residues
uN + v, or as
=
Grams of Amino Acid N
pN + q, Respectively, in 100 g of Seed Dry Matter
u
species
wheat
± SD
triticale
rye'
barley
corn
sorghum
pearl millet'
foxtail millet'
6372± 208
rice
oats'
5650
5550
5122
5253
5552
5366
4789
5142
pea
lupin
soybean
broad bean
French bean
sunflower
±
±
±
±
±
± SD
r
-670 ± 150
-422 ± 261
-143 ±511
998
996
994
999
999
998
998
994
997
990
997
999
997
v
5590° ± 59
5375 ± 115
5174 ± 176
5195 ±81
5880 ± 50
5550 ± 112
5159 ± 69
130 ± 206
-720 ± 90
-87 ± 245
21 ± 154
-1381 ± 524
-748 ± 271
-767 ± 329
583 ± 260
128 ± 360
-1072 ± 1257
167
128
66
61
224
p ± SD
989
959
292
904
1001
955
917
1055
1020
1005
1003
1030
1024
±
±
±
±
±
±
±
±
±
±
±
±
±
10
16
23
12
8
21
11
37
20
18
12
14
41
q ±
SD
-72 ± 25
-78 ± 36
-30 ± 66
30 ±31
-100 ± 13
-14 ± 45
41 ± 26
-158 ±
-136 ±
-137 ±
-204 ±
-392 ±
-469 ±
93
33
46
49
85
232
ref
r
l6
999
998
997
999
999
998
999
993
999
994
998
998
997
2
11
3
4
5
9
10
6
9
7
8
9
-1024
9
9
1404
-435
9
All the figures of the table have been multiplied by 103. See Table I, footnote c. Species for which total AA nitrogen
by taking into account the NH3 eluted with AAs instead of that resulting from amide groups only.
b
0
c
was
determined
theoretical interest only and was not considered in the present
paper. As for the accurate factor k allowing determination of
the accurate content of actual seed proteins from N, it can only
be approached with an improved accuracy in a way suggested
at the end of this paper.
RESULTS AND DISCUSSION
Variation of the Conversion Factors as a Function of Seed Nitrogen Content. In recent papers dealing with AA composition of 10 cereal and legume species
and listed in Table I, we have shown that the level of
AA (i.e., each of the 20 AAs that comeach common
occur in protein molecules) in seed dry matter
increases linearly with N. It follows that their total contents
(as grams of AA residues/100 g of seed dry
matter) and
(as grams of AA N/100 g of seed dry
matter) also increase as linear functions of N described
monly
by
y^Ej
=
uN +
y2Di
=
PN +
(3)
q
(4)
in which the coefficients , , , and q are constant within
The values of these coefficients are reported
in Table II with corresponding standard deviations (SD)
and correlation coefficients r. It is worth noting that r
values are close to 1 and all are higher than 0.99, which
corresponds to a significance level equal to 10~4 or 10~5
for most of them. This allows assessment of the conversion factors defined above:
a species.
kA
=
(uN + u)/(pN + q)
kp
=
u
+
/
(5)
ter basis) for corn (left side) and pea (right side). The plausible deviation for k is represented by the striped area.
in Table II also show that, for all the species, the variations of both factors are always more significant at low,
than at high, N values. Figure 1 suggests that for a given
N value kA is higher than kP, which is obvious. Since
corresponds to total AA N only, while N corresponds to total (AA plus nonprotein) seed N, it follows
< N. As a consequence, £;/ '
> Y,EJN.
that
That
is
kA > kP
It can also be noted that the N recovery (R) in A A analyses, which equals the ratio of AA N to total seed N (R
=
Y.Di/N) is always <1. According to eq 1 and 2
(6)
This shows that both factors change as quadratic functions of N represented by segments of equilateral hyperbola. According to the species and to the kind of factor,
the latter can either increase of decrease as a function of
N, that is, as a first approximation, as a function of total
seed protein content. As examples, the variations of kA
and fcP for corn and pea are drawn in Figure 1. This figure shows that for pea both kinds of factors decrease as
a function of N. The same is true for lupin seed (Mossé
et al., 1987), while kP slightly increases contrary to kA
for soybean. For all cereals except barley, kP increases,
whereas kA can either remain constant or increase according to the species. Equations and 6 and data reported
(7)
kP
=
RkA
(8)
means that kP is lower than kA.
Comparison with Literature Data. Although it is
devoid of difficulty, calculation of the conversion factors
is sophisticated enough to result in some miscalculation.
The most frequent but minor error in the literature consists of taking into account the weight of amide (or analytical) NH3 in the calculation of total weight ( - ;) °f
AA residues, as did Heathcote (1950) and Tkachuk (1966a,
1977) who were the first to improve the method of conversion factor determination by using AA compositions,
as did Morr (1981). Another kind of error
results from
confusion between total weight of AAs and total weight
of their residues. Therefore, the values of kA deter-
which
N-Protein Conversion Factor for Grains
J. Agrie.
Food Chem., Vol. 38, No.
1,
1990
21
Table III. Comparison between Literature Values* (LV) and Present Work (PW) for N Recovery R from AA Analysis and
for Conversion Factors kA and kP
% R
species
wheat
triticale
rye
barley
pearl millet
oats
pea
lupin
soybean
broad bean
French bean
°
K
kp
»
LV
PW
LV
PW
% Ac
LV
PW
% Ac
2.94
2.86
3.15
2.53
2.14
2.37
3.13
4.57
6.17
6.18
4.47
4.61
95.8
96.3
95.3
89.6
90.5
98.4
92.7
96.4
96.4
93.4
91.7
91.8
93.4
96.1
95.8
96.6
94.8
n.d.
n.d.
5.37
5.40
5.49
5.05
5.13
5.59
5.10
5.25
4.94
5.22
5.03
5.11
5.36
5.36
5.24
5.12
5.26
5.17
5.31
5.25
5.27
5.38
5.13
5.09
0.2
0.7
4.8
5.61
5.61
5.76
5.63
5.66
5.68
5.50
5.52
5.32
5.63
5.50
5.71
5.56
5.56
5.61
5.58
5.72
5.53
5.52
5.48
5.46
5.67
0.9
0.9
2.7
0.9
95.1
92.9
92.8
91.4
89.4
-1.4
-2.5
8.1
-3.6
0
-6.3
-3.0
-2.0
0.4
-1.0
2.7
-0.4
-1.0
-2.6
-0.7
6
According to, or calculated from, data of Tkachuk (1969) for cereals and Sosulski and Holt (1980) for legumes. Seed nitrogen percentc
basis.
Relative
difference
work
and
literature
values
as
the
between
dry
present
expressed
percentage 100(LV
PW)/PW.
age on a
mined in a first step by Morr (1981) for several soybean
protein products have been ca. 18% overestimated. Afterward, they were correctly recalculated (Morr, 1982). The
data reported in Table II show that for soybean kA
decreases from 5.78 to 5.63 when N increases from 4.5 to
7.3. These values agree with those of Morr (1982) as well
as with those that can be worked out from AA compositions published by Smith and Circle (1972): All these determinations of kA for selected soy protein products actually range between 5.63 and 5.80.
The values of kA (or kA) and kP obtained from Table
II for a given N can be compared with published data or
calculated from data available in the literature. Such a
comparison is shown in Table III for cereals investigated
by Tkachuk and Irvine (1969) and Tkachuk (1969) and
for legume seeds analyzed by Holt and Sosulski (1979)
and Sosulski and Holt (1980). In most cases the agreement is striking. For kA (or kA) the relative deviations
between our results and those from these authors are less
than, or close to, 1 %. The agreement is also good for kP
of which determination depends on R, as discussed later
on. The relative deviations are greater than ±5% for
pearl millet and for lupin only. They can be easily
explained. Although the millet sample has been analyzed with great care by Irvine and Tkachuk (1979), the
N recovery (R) of this sample is still abnormally high:
98.4%, instead of 93.4% in the present work, while that
of the average of the two lupin samples is low (92.9%
instead of 96.6%). This plausibly results from an imperfect calibration in AA determination leading to some overestimation (or underestimation for lupin) of total weight
Y,E¡ of residues. For legume seeds, Sosulski and Holt
(1980) performed only one 24-h acid hydrolysis (instead
of three) plus particular hydrolyses for S AAs, tryptophan, and amide N. For broad bean, pea, and French
bean they published average AA compositions of 6, 17,
and 4 samples, respectively, which plausibly explains the
reliability of their results. They analyzed only two lupin
seed samples: This might be the cause of discrepancies
with the present results. The same is true for Ewart (1967)
who determined kA in single samples of five cereal species after hydrolysis conditions close to those of Sosulski and Holt (1986). His results (not reported in Table
III) are 2.5 ±1% higher than those of the present work.
Significance of the Factors kA and kP. It is worth
noting that kA obtained from AA composition is a N:P
conversion factor—and the true one—for a pure protein
only or for mixtures in which there are no other N compounds than proteins. Its reverse, 100/fcA, provides the
N percentage in the protein(s) concerned. From a theoretical standpoint, this percent N varies from 8.58 for
-
AMINO ACID
RESIDUES AND IN
SOME PROTEINS
N % IN
—
30
25—
20—
TUNA FISH PROTAMINE
His
BOV. HISTONE IIB,
—
Gly, Asn
—
Lys, Gin
BOV. AND PEA HIST. IV
HUM.
LYSOZYME
CHICKEN J
PIG
RNASE
Ala
BOV.
J
HUM. IG y, (EU)
aS,
CASEIN
P
J
LACTOGLOBULIN
lie
-Tyr
5—
Figure 2. Percent N in the
proteins. Key: Bov
=
AA residues and in
common
bovine; Hum
=
a
few
human.
polytyrosine to 35.87 for polyarginine. Its values in common AA residues are displayed in Figure 2 with those
ones of some proteins. The latter were selected from those
of which the AA sequence is known (Dayhoff, 1972), that
is the proteins in which accurate amounts of each of the
four AAs have been determined: asparagine, aspartic acid,
glutamine, and glutamic acid. This figure shows that AAs
are distributed in three groups according to percent N
in their residues. Tyrosine, phenylalanine, methionine,
and glutamic acid residues are N poor (from 8.58 to
10.85%) while lysine, glutamine, asparagine, glycine, histidine, and arginine residues are N rich (from 21.86 to
35.87%). For the other 10 AA residues, percent N ranges
within 16 ± 4% (from 12.17% for aspartic acid to 19.7%
for alanine). It is thus unlikely that percent N in a pure
protein, and all the more so in a protein mixture, be less
than 15% or more than 30%.
According to eq 1 this percent N in total seed proteins
equals 100 / ^ · Therefore, its variations as a function of total seed N are represented by segments of equi-
22
J. Agrie. Food Chem., Vol. 38, No. 1, 1990
Figure
3.
Mossé
Variation in the percent N in total and actual seed proteins
as a
function of seed N percentage N for several cereal and
legumes.
lateral hyperbola drawn in Figure 3. It significantly
decreases as a function of N for wheat, corn, and particularly foxtail millet. It is virtually constant for barley,
triticale, rye, rice, and oats, whereas it more or less significantly increases for the three legume seeds represented. In any case, it remains within the range 17.9 ±
1%.
For total storage proteins accumulated in different
amounts in seeds according to N (Mossé et al., 1986,1988b,
1989; Huet et al., 1987), it is possible to calculate their
percent N equal to 100p/u. Surprisingly, despite the fact
that they consist of mixtures of numerous polypeptide
chains (plausibly more than 100 or so for cereal grains),
the percent N of seed storage proteins differs significantly according to the species and ranges from 16.6%
for foxtail millet to 19.6% for pea and lupin.
An advantage of kA (or of its reverse) determination
is that, according to eq 1, it is practically independent
of N recovery in AA analyses, provided that R does not
have an abnormal value. In fact, R is the product of two
terms, R1 and R2, each of which is lower than unity. R1
is the actual analytical recovery of AAs expressed here
as percent N. Due to slight alterations or losses of minute
amounts of AAs, mainly during hydrolysis, R1 < 1. Howand
ever, such losses result in a similar decrease of
without significant changes in their ratio kA. This
can still be a little less valuable for kA for which analytical NH3 is used instead of amide NH3. As for R2, it is
the ratio of total common AA N to total seed N, and 1
R2 indicates the NPN amount. Outside of mixtures of
pure proteins (where R2 = 1), R2 < 1. This distinction
between R1 and R2 has been neglected until now in the
careful studies of both Holt and Sosulski (1979) and Sosulski and Holt (1980) as well as by myself in the papers
cited in Table I. Though Rx and R2 are probably close
to each other, their accurate determination is practically
impossible. Several attempts have tried to determine
R2, but they rely on extraction techniques that cannot
be quantitative enough and only give an approximate magnitude (Holt and Sosulski, 1981; Baudet et al., 1986a).
As suggested by Teller (1932), Heathcote (1950), Tkachuk (1969,1977), and Sosulski and Holt (1980), kA must
be corrected because it does not make allowance for NPN.
Therefore, it does not allow calculation of the real pro-
tein amount from total seed N. However, in any case, it
must be emphasized that kA is a little higher than the
upper limit of the true conversion factor, k < kA.
On the other hand, outside industrial food protein products, kA has little use in practice. For instance, the present
work shows that kA is close to 5.7 for soybean proteins
and this value is the real conversion factor for purified
soy protein isolates.
kP has been named the “corrected conversion factor”
by Sosulski and Holt (1980) who are the first to have
published values of both kinds of conversion factors. kP
has indeed the advantage of taking into account NPN.
This is a reason explaining eq 7 and results from the fact
that R2 < 1. However, eq 7 also results from the fact
that kp depends on analytical recovery {R1 < 1) of AAs
during their analysis. Indirect evidence shows that both
R1 and R2 slowly increase with N in most species (unpublished results). As a consequence, the true conversion
factor k is a little higher than kP, which corresponds to
could
the lowest limit that may be reached by k (if
equal 100%). In other words, k is always within a range
narrower
than that limited by kp and kA.
kA < k < kp
(9)
Assessment of the True N:P Conversion Factor Jr.
Calculation from data reported in Table II shows that,
according to the species investigated, the relative difference 100(feA
kp)/0.5(kA + kp) generally decreases as a
function of N and ranges between 6 and 10% at low N
and between 1.5 and 4% at high N of the species concerned. This leads to the assumption that k is very close
to the average of kA and kP with possible deviations that
plausibly do not exceed ±(/zA kP)/4 as it is represented
for corn and pea in Figure 1. Such an assumption allows
quantification of k as a function of N by
-
-
k
=
(kA +
kp)/2±(kA-kp)/4
(10)
function of N according to
The variations of k as a
such a definition are represented in Figure 4 for the cereals and legumes investigated here. For rye, pearl millet
(not shown because its k variations are close to those of
rye), barley, soybean, and sorghum, k can be considered
as constant and virtually independent of N. For pea and
lupin it decreases about 0.1 unit within the N range of
N-Protein Conversion Factor for Grains
Figure
4.
Variation in the N to protein conversion factor k
J. Agrie.
as a
these seeds. On the contrary, for wheat, corn, foxtail millet, oats, and rice, it significantly increases with N. Its
highest variations
occur
in foxtail millet for which it
increases from 5.28 for N = 1% to 5.98 for N = 4%.
The present results also show that for cereals k variations from N = 1 to 2% are roughly twice as much as
they are from N = 2 to 4%. Since, in developed countries, the most frequent N value is >2 for many cereals,
it can appear useless to take into account data on the
variation of k according to N. However, for rice, the most
frequent N value is around 1.2 (for many Asian rice samples N ranges from 0.8 to 1.5%) and this variation is really
significant. On the other hand, it must be kept in mind
that N is determined on aliquots of tremendous numbers of grains. For instance, on the world scale according to FAO (1988), the average yield of wheat is now higher
than 2 metric tons/ha (i.e., per 2.5 acres), that is more
than 50 million wheat grains/ha. Therefore, N is an average value of widely N ranged grains. As new processes
for screening grains according to N on silo scale are going
to emerge in the future, this may render knowledge of k
variation according to N useful.
A simplified way still consists of the use of averaged
values of k for the species investigated. These approximate values are, in decreasing order as follows: foxtail
millet, 5.8 ± 0.2; corn, 5.65 ± 0.15; sorghum, 5.65 ± 0.02;
soybean, 5.52 tu 0.02; barley, 5.50 ± 0.02; pea, 5.44 ±
0.14; lupin, 5.40 ± 0.08; triticale and oats, 5.36 ± 0.05;
wheat, 5.33 ± 0.17; pearl millet, 5.33 ± 0.05; rye, 5.33 ±
0.03; rice, 5.17 ± 0.25. From this viewpoint, it is possible to calculate k for other legumes and for single samples (corresponding to single N values) of oilseeds analyzed by Sosulski and Holt (1980) and Tkachuk (1969).
Calculation gives k = 5.40 for broad bean, 5.33 for French
bean, and 5.30 for rapeseed and sunflower samples (with
N close to 5% in nondefatted oilseeds). The present results
show that k varies from 5.13 for N-poor rice samples to
about 6.0 for N-rich foxtail millet samples. It is thus
true that substantial differences can reach 20% in protein content for grains corresponding to equal N as noted
by Heidelbaugh et al. (1975) when these authors inves-
Food Chem., Vol. 38, No.
1,
1990
23
function of N for several cereal and legumes.
tigated the food system involved in the skylab manned
space flight program.
On an average between species, the (true) conversion
factor k is often close to 5.4 or 5.5, rather than 5.7 or
6.25, as generally admitted. However, as compensation,
the lower k is, the higher the AA content in seed proteins. As an example, with the conventional factor 6.25
(which implies that 100 g of protein correspond to 100/
6.25 = 16 g of N) for N = 2%, lysine content has been
shown to equal 2.6 g/16 g of N for corn and 2.9 for wheat
(Baudet et al., 1986; Mossé et al., 1985). The present
work shows that these contents equal 2.9 g/100 g of actual
proteins for corn and 3.35 for wheat.
ACKNOWLEDGMENT
I thank J.
racy of
C.
numerous
Huet for his help in checking the
accu-
calculations.
Registry No. N2, 7727-37-9.
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Received for review January 17,1989. Accepted May 16,1989.